1. Field of the Invention
The present invention relates to a process for producing an SOI-structured semiconductor device substrate by bonding method and more particularly to a process for producing an SOI-structured semiconductor device substrate in which a thin semiconductor monocrystal deposition film is formed on an insulating layer.
2. Description of the Related Art
In order to facilitate a device isolation in an integrated circuit or in particular, in order to overcome latchup phenomenon in a CMOS semiconductor circuit, an SOI-structured semiconductor device substrate has been provided.
In order to provide such SOI-structured semiconductor device, a process forming an oxide film constituting an insulating film on Si-monocrystal wafer and then depositing a polycrystal layer on the insulating film which is then made into a monocrystal thin film by laser etc. or a process pyrolytically depositing an Si-monocrystal thin film on sapphire substrate from a gaseous Si compound have been employed.
However, since the crystallinity of Si-monocrystal thin film on insulating film as deposited or laser fused and solidified after deposit by these prior-art processes has not been satisfied for the use of semiconductor devices, further processes have been developed. In particular, a process in which a Si-monocrystal wafer is bonded to the top surface of an insulating thin film provided on a semiconductor substrate and then the bonded Si-monocrystal wafer is thinned into a desired thin film by polishing or etching has prevailed.
This bonding process is categorized into processes using a simple weight and an electrostatic pressure in order to press the Si-monocrystal wafer to the top surface of the insulating thin film.
Unexamined Japanese patent application publication No. SHO 48-40372 discloses the weight bonding process, for example. The teaching of this publication is that a Si-monocrystal wafer is overlaid on an oxide film and bonded thereto at 1,100.degree. C. or more under a pressure of 100 kg/cm.sup.2 or more.
Pages 92-98 of "NIKKEI MICRODEVICE" published Mar. 1, 1988 by Nikkei McGraw-Hill CO. disclose the electrostatic pressure using bonding process, for example. This process will be described hereinafter.
FIG. 4(D) illustrates an example of SOI-structured semiconductor device substrates produced by electrostatic pressure bonding process. In the FIG. 4 process, there is shown the bonding of wafer 1a and wafer 1b to each other with oxide film 1c constituting an insulating film provided therebetween and then surface-grinding and/or etching the exposed top surface of the wafer 1b so that the wafer 1b is made a thin film of an SOI-structured semiconductor device substrate. In further detail, this SOI-structured semiconductor device substrate is explained as follows:
First, as shown in FIG. 4(A), thermally oxidizing the overall surface of each of the wafers 1a and 1b forms an oxide film 1c, e.g., with a 0.8 .mu.m thickness. Then, as shown in FIG. 4(B), the wafer 1b is overlaid on the wafer 1a. The assembly of the wafers 1a and 1b is placed in a furnace. A pulsating direct or alternating voltage at about 300 V is applied to the assembly of the wafers 1a and 1b across the total thickness under the atmosphere of N.sub.2 at about 500.degree. C. Thereby, the wafers 1a and 1b are bonded. A bonding between the wafers 1a and 1b is strong so that the bonded wafers of the wafers 1a and 1b can be put into a conventional thinning process. Then, as shown in FIG. 4(C), the overall oxide film 1c formed on the top and cylindrical surfaces of the wafer 1b and a part of the wafer 1b underlying the oxide film 1c formed on the top are eliminated by surface-grinding or etching so that the thickness of the wafer 1b is uniformly reduced to a predetermined thickness over the wafer 1b. Then, as shown in FIG. 4(D), specularly polishing the resulting top surface of the wafer 1b at least to the extent of no work damage leftover therein from previous processes provides a monocrystal thin layer of SOI-structured substrate for forming a semiconductor device therein.
A polishing step will be described hereinafter.
FIGS. 2 and 3 illustrate the main part of a single-side polishing machine. In FIGS. 2 and 3, a wafer mount plate is indicated at 2. The underside of the wafer mount plate 2 has a plurality of wafers 1 which have been surface-ground and alkali-etched and detachably attached thereto by means of wax. On the other hand, the top surface of a turntable 3 provided under the wafer mount plate 2 has a polishing cloth 3a attached thereto. The single-side polishing machine press down the wafers 1 on the polishing cloth 3a by means of the wafer mount plate 2 so that the undersides of the wafers 1 attached to the underside of the wafer mount plate 2 are in contact with the polishing cloth 3a, and the wafer mount plate 2 is rotated in response to the rotation of the turntable 3. During polishing operation, a suspension with abrasive grains in the form of an abrasive slurry and controlled alkalescent by NaOH or NH.sub.4 OH, abrasive grains being such as colloidal silica, is fed onto the polishing cloth 3a.
However, the above-described electrostatic pressure using bonding process entails the following problems. The bond strength of the bonded wafers of the wafers 1a and 1b with the oxide film 1c therebetween is low at their periphery, so that a part of the periphery of the wafer 1b may be chipped off during a surface grinding of the wafer 1b. In this case, the oxide film 1c on the cylindrical surface of the wafer 1b is also lost as shown in FIG. 4(C). Thus, when the single-side polishing machine polishes the top surface of the wafer 1b of the bonded wafer 1 after the surface grinding, some parts of the periphery of the top surface of the wafer 1b with their adjoining parts of the cylindrical surface lacking the oxide film 1c are ready to be more polished than the remaining part of the periphery of the top surface of the wafer 1b. In particular, when the wafer 1b is made a thin film with 5 .mu.m or less thickness, a thickness control of the resulting thin film is difficult for the above mentioned reason.
In addition, the single-side polishing machine causes an elastic deformation in the wafer mount plate 2 by means of pushing each of the wafers 1 against the polishing cloth 3a and has native differences in the peripheral speeds of radially different positions of the turntable 3 thereby to produce fluctuation in the thickness across the wafer 1b which has been polished.
Therefore, an object of the present invention is to provide a process for producing a semiconductor device substrate in which controlling a polishing amount of a wafer is easier and a variation in the thickness of the wafer can be controlled lower.
The above and other objects, and advantages of the present invention will be apparent from the attached claims and following description with reference to the drawings.